1. Field of the Invention
The present invention relates to an image recording apparatus for recording an image by guiding a light beam to a recording medium and moving the light beam and the recording medium relatively to each other to scan the recording medium with the light beam in main and auxiliary scanning directions.
2. Description of the Related Art
There have heretofore been known image recording apparatus in which a light beam emitted from a semiconductor laser is focused by a focusing optical system into a focal point on a photosensitive recording medium which is wound around the outer circumferential surface of a drum, and the drum is rotated at a high speed about its own axis in a main scanning direction while the light beam is deflected along the axis of the drum in an auxiliary scanning direction, for thereby recording an image on the photosensitive recording medium.
In the known image recording apparatus, if the rotating drum has its outer circumferential surface not concentric with its axis, the photosensitive recording medium is lifted off the drum, or the auxiliary scanning direction in which the light beam is deflected does not extend along the axis of the drum, then the light beam is not focused highly accurately onto the photosensitive recording medium, failing to produce a highly accurate image on the photosensitive recording medium. Particularly when a halftone dot image is to be formed on the photosensitive recording medium by way of area modulation, it is necessary to focus the light beam highly accurately onto the photosensitive recording medium because the light beam needs to be constricted to a very small beam spot.
In an attempt to alleviate the above problems for focusing the light beam accurately onto the scanned surface of the photosensitive recording medium at all times, there has been employed an automatic focusing mechanism for moving the focal point of the focusing optical system depending on the distance or displacement between the scanned surface and the focusing optical system. The automatic focusing mechanism controls the focusing optical system in order to position the scanned surface within a desired depth of focus.
The automatic focusing mechanism is capable of eliminating out-of-focus states due to positional fluctuations of the scanned surface. However, the automatic focusing mechanism suffers limitations because its principles are based on moving the focal point of the focusing optical system. In addition, the automatic focusing mechanism lacks reliability as it is unable to handle a plurality of light beams.
It is a general object of the present invention to provide an image recording apparatus which is of a relatively inexpensive construction and is capable of focusing a light beam from a light source highly accurately onto a recording medium so as to record a highly accurate image thereon regardless of positional fluctuations of the recording medium.
A principal object of the present invention is to provide an image recording apparatus which is capable of generating a plurality of focused positions of a light beam substantially symmetrically in front of and behind a recording medium for thereby virtually increasing the depth of focus.
Another object of the present invention is to provide an image recording apparatus which is capable of suppressing density variations of an area-modulated image regardless of intensity fluctuations of a light beam and positional fluctuations of a recording medium.
Still another object of the present invention is to provide an image recording apparatus which is capable of automatically adjusting the distribution of beam intensities at focused positions generated in front of and behind a recording medium.
Yet another object of the present invention is to provide an image recording apparatus which is capable of generating a plurality of focused positions of a light beam in either a main scanning direction or an auxiliary scanning direction with respect to a recording medium, for thereby increasing the depth of focus.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example.